A liquid-crystal display (“LCD”) may be considered a variable light filter. Liquid crystals are trapped between two sheets of glass and walled off from one another into image bits called pixels. Such crystals twist and untwist to let polarized light through, and filters placed in front of the pixels are used to create the resulting colors produced by the LCD. Since an LCD modifies light and does not create it, the visual image produced by the LCD typically must be backlit by light generated beneath and the LCD and projected in the direction thereof for the image to be visible or discernable to a user.
A cold-cathode fluorescent lamp (CCFL) is often used for backlighting an LCD. One problem that often occurs with the use of a CCFL is that a CCFL only produces an approximation of white light, not true white light. Since an LCD can only present to a user colors that lie within the spectrum of light received by the LCD, a CCFL-based LCD has a color gamut (the extent of the mix of color a display is capable of producing) that relatively small and produces only about 80% of the color gamut recommended by NTSC (National Television Standards Committee) specifications.
An LED (Light Emitting Diode) array may also be used for backlighting an LCD. One advantage of using an LED array for backlighting an LCD is that an LED array can be configured to generate a wider and more accurately tuned spectrum of light than a CCFL. For example, by mixing an appropriate amount of light from red, green, and blue LEDs white light can be generated by an LED array. In addition, since the white balance of an LED array can be adjusted by varying the relative intensity of the red, green, and blue LEDs, the color balance of an LCD may be maintained reasonably accurately over the operating life of the LCD.
Another advantage of using an LED array for backlighting an LCD is that an LED array may provide improved color saturation in the LCD. LEDs are also not as fragile as CCFLs and as a result are more durable. Maintaining the uniformity of light emitted by an LED array becomes progressively more difficult to achieve as the LEDs in an array age, especially since different LEDs in the array may change light emission characteristics at different rates. Also, the use of three separate light sources for red, green, and blue colors of light may cause a point of light appearing on the display to move as the light emission characteristics of the various individual LEDs in the array change at different rates.
LEDs used for backlighting an LCD are typically driven by a driving circuit comprising a current source with Pulse Width Modulation (“PWM”). Such an approach is acceptable for many applications. Other power modulation methods may be used, however.
PWM has several advantages. For example, an LED backlight on an LCD using a fixed DC voltage method may require a driving current of 120 ma to produce a typical brightness of 50 NIT (a unit of measurement of the intensity of visible light, where 1 NIT is equal to one candela per square meter). If instead of using a fixed DC voltage method PWM is used in conjunction with five times the current (600 ma) for ⅕ of the time, the average current remains the same (120 ma), while perceived brightness increases. The human eye has a certain amount of persistence. When exposed to bright light the eye “remembers” the light for a short period of time. This phenomenon results in a motion picture or TV screen being perceived as a steady image when in fact it is, by way of example, flickering at 24 to 50 times per second. When an LED is flashed on at a high level of brightness for a short period of time and then turned off, the eye “remembers” the light at the high brightness level. The result is that the perceived brightness of the backlight is closer to the higher pulsed PWM brightness than it is to a lower average DC brightness.
PWM may also be used to give a “normal” looking brightness level to the LCD by using lower average current to save power. For example, average power may be cut by about 30% while generating a perceived “normal” perceived brightness level. Another use of PWM is to facilitate brightness control for LED backlighting without causing backlighting to appear uneven. By varying the duty cycle of the controlling PWM waveform, a wide range of brightnesses can be achieved while maintaining substantially even appearing backlighting.
Reducing power consumption is an especially important concern in battery-powered portable electronic devices such as laptop computers, PDAs, mobile telephones and the like. As a result, a strong motivation exists to find ways to reduce power consumption in battery-powered portable electronic devices while maintaining device performance.
In battery-operated handheld or portable electronic devices such as smart phones, ultra-mobile personal computers)“UMPCs”), laptop computers, tablet PCs, and e-books, the amount of power required to drive displays in such devices typically consumes between about 25 and 40 percent of the overall power required to operate such devices. Moreover, in backlit LCD displays, LEDs or CCFLs consume the lion's share of the power required to power a backlit LCD. In some cases the relative proportions of power consumed by backlighting LEDs or CCFLs on the one hand, and LCDs on the other hand, are roughly 80 percent and 20 percent, respectively.
Accordingly, LEDs or CCFLs provided to backlight LCD displays consume a disproportionately large amount of the total amount of power required to operate a batter-operated portable electronic device. Ways to reduce power consumption in backlit LCDs in battery-operated handheld or portable electronic devices without sacrificing display performance are therefore desirable.